Michael Gass, first CEO of the U.S. military launch monopoly United Launch Alliance (ULA), will retire from the company at year’s end, just as it faces a potential competition for work with upstart Space Exploration Technologies (SpaceX).

He is being replaced by longtime Lockheed Martin executive Tory Bruno, effective immediately; Bruno has run Lockheed Martin’s Trident D-5 submarine-based nuclear missile program, as well as the company’s Terminal High Altitude Area Defense (Thaad) program. Gass and Bruno will "work collaboratively" to ease the transition, according to a ULA announcement.

"With my intent to retire in the near term and the changing industry landscape over the next several years, the board of directors and I have agreed that the immediate appointment of my successor to begin the leadership transition is in the best interest of the company," Gass said in an Aug. 12 statement.

Gass is stepping down as ULA faces unprecedented scrutiny over the cost of its launch services for the U.S. government. Since ULA was established in 2006—from a 50/50 partnership between Lockheed Martin and Boeing, legacy manufacturers of the Atlas V and Delta IV rockets, respectively—it has been the sole provider of these rocket services to the U.S. Air Force, Navy and National Reconnaissance Office. The pair merged to lower the high cost of launch for the U.S. government.

Meanwhile, the company is being challenged by SpaceX. The U.S. Air Force is now reviewing data from three SpaceX launches of its Falcon 9 v1.1 rocket, with the goal of getting it certified to compete against ULA’s Evolved Expendable Launch Vehicle (EELV) systems for U.S. government work.

The company is also up against pressure due to the deteriorating relationship between Washington and Moscow, prompting a full review of propulsion alternatives for the Atlas V—the RD180 engine is made in Russia and sold to ULA. Russian Deputy Prime Minister Dmitry Rogozin has suggested that Moscow could halt RD180 deliveries, though to date Russia has not made good on the threat.

U.S. government officials are weighing whether to start a new liquid oxygen hydrocarbon engine program to both invigorate industry here and reduce reliance on a Russian propulsion system.